In wavelength division multiplexing (WDM), a plurality of optical signals or channels are carried over a single optical fiber with each channel being assigned a particular wavelength. Optical signals or channels in a WDM system are frequently transmitted over silica-based optical fibers, which typically have relatively low loss at wavelengths within a range of 1525 nm to 1580 nm. WDM optical signal channels at wavelengths within this low loss “window” can be transmitted over distances of approximately 50-100 km without significant attenuation. For distances beyond 100 km, however, optical amplifiers are required to compensate for optical fiber loss.
Optical amplifiers have been developed which include an optical fiber doped with erbium. The erbium-doped fiber is “pumped” with light at a selected wavelength, e.g., 980 nm, to provide amplification or gain at wavelengths within the low loss window of the optical fiber. However, erbium doped fiber amplifiers (EDFA) do not uniformly amplify light within the spectral region of 1525 to 1580 nm. For example, an optical channel at a wavelength of 1540 nm is typically amplified 4 dB more than an optical channel at a wavelength of 1555 nm. While such a variation in gain can be tolerated for a system with only one optical amplifier, it cannot be tolerated for a system with plurality of optical amplifiers or numerous, narrowly-spaced optical channels. In which case, much of the pump power supplies energy for amplifying light at the high gain wavelengths rather than amplifying the low gain wavelengths. As a result, low gain wavelengths suffer excessive noise accumulation after propagating through several amplifiers.
Accordingly, dynamic gain equalizers (DGE) have been used to balance the gain across channels and provide a substantially uniform spectral gain, that is, provide gain flatness. DGEs work well for static optical network systems; however, optical network systems are not static and channel degrading and DGE operation can change with temperature, component drift, aging and fiber plant maintenance among other considerations. Accordingly, optical channel monitoring (OCM) is needed to ensure that the DGE provides the necessary gain flatness despite changing conditions.